(1) Field of the Invention
The present invention relates to a screw having a drive recess that is not easily stripped off when a driver bit is engaged with the drive recess and strong driving force is applied to the recess. Also, the present invention relates to a driver bit for use with the screw.
(2) Description of the Related Art
A screw having a drive recess, which consists of a screw head with the drive recess and a screw shank with threads, is used in every field as a fastening part for assembling a product, attaching parts to a workpiece and so on.
FIG. 1 is a cutaway view showing a shape of a drive recess 110 of a conventional screw 101 having a drive recess (a screw having a drive recess is hereinafter simply called the “screw”).
The drive recess 110, which is substantially in cross shape as seen in plan view, is formed on top of a screw head 102 of the screw 101. A drive head of a driver bit is to be inserted into the driver recess 110 in such a manner that an engaging blade, which is formed on the drive head of the driver bit, is engaged with an engaging groove 111. As the driver bit is inserted and driven rotatively, the screw 101 is fastened to a workpiece.
Recently, with the miniaturization of workpieces and products, screws used therefor are becoming comparatively smaller.
If a screw becomes smaller, the drive recess 110 thereof correspondingly becomes smaller. It results that the engaging groove 111 of the driving recess 110 also becomes smaller, and it sometimes happens that the engaging groove 111 can not withstand strong driving force and therefore stripped off, and it becomes impossible to fasten the screw 101 tight.
To solve such a problem, for instance, the engaging portion may be made larger as much as possible by deepening the drive recess 110. However, when the drive recess is deepened, the bottom of the drive recess 110 gets into the screw shank, and the portion around the junction of the screw head and the screw shank (called a neck portion) becomes thinner. As a result, the mechanical strength around the neck portion is deteriorated and the screw head might come off, which is called “head come-off” phenomenon.